The present invention relates to a method for correcting digitized image files in the prepress stage when using printing presses having a device-independent color profile in the main printing stage.
Basically, a printing process is divided into three fields: prepress stage, main printing stage, and print finishing. The first two fields are primarily of interest for the present invention, all work steps before the actual printing being included in the prepress stage. These include, for example, the typesetting, i.e., the text acquisition, the text formatting, and the text make-up, the reproduction of artwork and graphics, and the paste-up and printing form production. Up to the transfer onto the printing form(s), the prepress work processes are frequently implemented currently as digitized work processes (digital workflow). The digital data is then applied directly to the printing form in the printing press using a plate imager or, in offset printing presses, using DI technology. The finished printed products of the printing press are to correspond as exactly as possible to the expectations of the client in this case, these client expectations often manifesting themselves in a reference sheet. This means that the data of the prepress stage is to be constituted in such a way that when it has been written on the printing form and the printing press operates using this printing form, printed products of the press are to be as similar as possible to the reference sheets.
Many approaches to achieve this object are known from the related art. Such a method arises from German Patent Application No. 100 56 723 A1, which allows a reference sheet to be compared with the associated printed products of multiple printing presses and the printing presses to be controlled in such a way that they largely provide identical printed products. For this purpose, a reference sheet is scanned in and saved in a standardized color space, e.g., a Lab color space or an RGB color space, in a device-independent representation. This device-independent data is then converted into a CMYK color space suitable for printing presses, and a sample print is then executed on each of the printing presses. The sample prints are scanned in turn, saved in a standardized color space, and compared to the device-independent data of the scanned reference sheet. If deviations arise in this case, these deviations are converted into the CMYK color space in the standardized color space and the non-standardized CMYK data is thus corrected. This correction is performed separately for each printing press, so that finally a transformation step is developed for each printing press which ensures that the printed products on the different printing presses have the appearance of the reference sheet.
The method described has the disadvantage that it is dependent on sample prints and thus produces waste, since, for an informative sample print, a specific number of sheets must be printed until the state of a printing press has stabilized.